1. Field of the Invention
This invention relates to a novel method for stabilizing extra-chromosomal elements in bacteria during cultivation, without the need for addition of antibiotics or other undesirable components to the growth medium, with the purpose of increasing the yield or quality of fermentation products.
2. Description of the Background Art
Microorganisms harboring extra-chromosomal genetic elements, for example plasmids, are, generally speaking, unstable in the sense that the element may frequently be irreversibly lost. This instability is particularly pronounced if the plasmid is not endogenous to the host microorganism, for instance, because it comprises genes from other organisms or has been constructed by gene splicing.
To increase plasmid stability, an antibiotic or another bioactive compound to which the plasmid but not the chromosome confers resistance, is commonly added to the medium used for cultivation of the microorganism. In such a medium only those cells retaining the plasmid with the antibiotic resistance gene proliferate. The major disadvantages of this approach are that it requires large scale growth of antibiotic resistant microorganisms, addition of expensive antibiotics to the growth medium with possible adverse effects on the environment, and subsequent extensive purification to remove the antibiotic from the desired product.
Complementation of an auxotrophic mutation of the host chromosome is another known method for stabilization of plasmids. This approach, however, seriously restricts the composition of the growth medium and requires cultivation in a growth medium that does not contain the nutrient required by the host microorganism, thereby limiting the options available for improving productivity.
The object of the present invention is to provide a method for stabilization of extra-chromosomal elements in transformed bacteria, without the necessity of using antibiotics and without severe restrictions on the composition of the growth media.
It is another object of the present invention to provide stabilized extra-chromosomal elements and transformed bacteria containing such stabilized extra-chromosomal elements.
It is a further object of the present invention to provide a method for producing desirable products in transformed bacteria without the necessity of using antibiotics or special restrictions on the composition of the growth media.
Other objects of the invention will become apparent to those skilled in the art to which it pertains.